Hide the sign-in link beside the wordmark

uhhm.no is read and answered without an account, so the invitation to
sign in has no audience. Portal's header nav keeps it next to the
wordmark on every page; a skin hides it, as on redoal.com.

The rule only takes the link in: signed in, the nav still carries the
name and the way out, and the gate on /review still offers its own way
in. First custom.css here, wired through site.yaml's stylesheet key.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015GeH3y8fwTaTudaD88P57o

site.yaml

@@ -5,6 +5,9 @@ hero:
   kind: module
   module: hero.js
 
+# The skin over portal's own stylesheet.
+stylesheet: custom.css
+
 # Mail goes out through gdo on this host, which sends over JMAP as
 # bl@uhhm.no; replies come back to the same address.
 mail: